Ammonia storage control

ABSTRACT

Various methods for controlling ammonia levels stored in a catalyst by controlling exhaust gas temperatures are provided. In one embodiment, a temperature of a catalyst in an internal combustion engine is determined. If the temperature of the catalyst exceeds a first threshold at which an ammonia capacity of the catalyst for the temperature is below a current stored ammonia level in the catalyst, a load of the engine is reduced including adjusting a torque output of a motor operatively coupled to the engine.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to German Patent Application No. 102012212051.3, filed on Jul. 11, 2012, the entire contents of which are hereby incorporated by reference for all purposes.

FIELD

The disclosure relates to a method for controlling ammonia levels stored in a catalyst of an internal combustion engine.

BACKGROUND AND SUMMARY

Internal combustion engines may include one or more catalysts configured to reduce the level of emissions produced during fuel combustion. For example, a selective catalytic reduction (SCR) catalyst may be used to reduce levels of nitrogen oxides (NOx) emitted following fuel combustion. The SCR catalyst may use one or more reductants to enhance NOx conversion, such as ammonia. For a given temperature, the SCR catalyst has a storage capacity which determines the level of reductant which may be stored in the catalyst. Exhaust gas temperatures, and the temperature of the SCR catalyst, may be controlled to ensure that a sufficient level of reductant is present in the catalyst to satisfy NOx conversion demands.

In some approaches, aqueous urea is stored in a tank and injected upstream of an SCR catalyst in the presence of sufficiently high exhaust gas temperatures which prompt conversion of the urea into ammonia. In the presence of the SCR catalyst, the ammonia may be used to reduce NOx levels. Following conversion from urea, ammonia may be stored in the SCR catalyst such that ammonia may be available for NOx conversion when exhaust gas temperatures are insufficient to facilitate urea conversion—for example, during a cold start.

The inventors herein have recognized a problem with such approaches. For example, ammonia stored in the SCR catalyst may be released to the ambient environment abruptly and unintentionally if the torque demanded by a vehicle operator is suddenly increased. Ammonia slip in this example may be caused by a sharp rise in exhaust gas temperatures due to the increased driver-demanded torque, as these temperatures have exceeded those at which ammonia storage in the catalyst is ensured. The ammonia slip in turn causes the increased emission of NOx into the ambient environment.

Various methods for controlling ammonia levels stored in a catalyst by controlling exhaust gas temperatures are provided.

In one example, a temperature of a catalyst in an internal combustion engine is determined. If the temperature of the catalyst exceeds a first threshold at which an ammonia capacity of the catalyst for the temperature is below a current stored ammonia level in the catalyst, a load of the engine is reduced including adjusting a torque output of a motor operatively coupled to the engine.

In this way, ammonia may be supplied to the catalyst to ensure sufficient NOx conversion while preventing ammonia slip and release to the ambient environment. Thus, the technical result is achieved by such actions.

The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.

It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a vehicle with a hybrid propulsion system.

FIG. 2 shows a block diagram of turbocharged engine.

FIG. 3 shows a flow chart illustrating a method for controlling a level of ammonia stored in a catalyst.

FIG. 4 shows an exemplary graph of the ammonia stored in the catalyst of FIG. 3 as a function of temperature.

DETAILED DESCRIPTION

In some internal combustion engines, urea is introduced upstream of a catalyst and, in the presence of sufficient temperatures, converted to ammonia. The ammonia is utilized as a reducing agent (e.g., reductant) to facilitate the conversion of NOx at the catalyst to other compounds. Several challenges arise in such approaches, however. Because the conversion of urea to ammonia occurs does not occur without sufficient surrounding temperatures, such conversion cannot occur at certain times of engine operation (e.g., a cold start). To compensate, ammonia may be stored in a catalyst and used during these times when conversion is not possible. Accurate control of exhaust gas temperatures, and the temperature of the catalyst, is required, however, to ensure sufficient conversion of NOx and that these temperatures do not exceed a level at which ammonia is released to the ambient environment (e.g., ammonia slip).

Various methods for controlling ammonia levels stored in a catalyst by controlling exhaust gas temperatures are provided. In one embodiment, a temperature of a catalyst in an internal combustion engine is determined. If the temperature of the catalyst exceeds a first threshold at which an ammonia capacity of the catalyst for the temperature is below a current stored ammonia level in the catalyst, a load of the engine is reduced including adjusting a torque output of a motor operatively coupled to the engine. FIG. 1 schematically shows a vehicle with a hybrid propulsion system. FIG. 2 shows a block diagram of turbocharged engine. The engine of FIG. 2 includes a controller configured to carry out the method depicted in FIG. 3. FIG. 4 shows an exemplary graph of the ammonia stored in the catalyst of FIG. 3 as a function of temperature.

Referring to FIG. 1, the figure schematically depicts a vehicle with a hybrid propulsion system 1. Hybrid propulsion system 1 includes an internal combustion engine 2, further described herein with particular reference to FIG. 2, coupled to transmission 3. Transmission 3 may be a manual transmission, automatic transmission, or combinations thereof. Further, various additional components may be included, such as a torque converter, and/or other gears such as a final drive unit, etc. Transmission 3 is shown coupled to drive wheel 4, which in turn is in contact with road surface 5.

In this example embodiment, the hybrid propulsion system also includes an energy conversion device 6, which may include a motor, a generator, among others and combinations thereof. The energy conversion device 6 may be drivingly coupled to engine 2 and drive wheel 4 via transmisson 3. The energy conversion device 6 is further shown coupled to an energy storage device 7, which may include a battery, a capacitor, a flywheel, a pressure vessel, etc. The energy conversion device can be operated to absorb energy from vehicle motion and/or the engine and convert the absorbed energy to an energy form suitable for storage by the energy storage device (i.e. provide a generator operation). The energy conversion device can also be operated to supply an output (power, work, supplemental torque, speed, etc.) to the drive wheels 4 and/or engine 2 (i.e. provide a motor operation). It should be appreciated that the energy conversion device may, in some embodiments, include only a motor, only a generator, or both a motor and generator, among various other components used for providing the appropriate conversion of energy between the energy storage device and the vehicle drive wheels and/or engine.

The depicted connections between engine 2, energy conversion device 6, transmission 3, and drive wheel 4 indicate transmission of mechanical energy from one component to another, whereas the connections between the energy conversion device and the energy storage device may indicate transmission of a variety of energy forms such as electrical, mechanical, etc. For example, torque may be transmitted from engine 2 to drive the vehicle drive wheels 4 via transmission 3. As described above energy storage device 6 may be configured to operate in a generator mode and/or a motor mode. In a generator mode, system 6 absorbs some or all of the output from engine 2 and/or transmission 3, which reduces the amount of drive output delivered to the drive wheel 4, or the amount of braking torque to the drive wheel 4. Such operation may be employed, for example, to achieve efficiency gains through regenerative braking, improved engine efficiency, etc. Further, the output received by the energy conversion device may be used to charge energy storage device 7. In motor mode, the energy conversion device may supply mechanical output to engine 2 and/or transmission 3, for example by using electrical energy stored in an electric battery.

Hybrid propulsion embodiments may include full hybrid systems, in which the vehicle can run on just the engine, just the energy conversion device (e.g. motor), or a combination of both. Assist or mild hybrid configurations may also be employed, in which the engine is the primary torque source, with the hybrid propulsion system acting to selectively deliver auxiliary added torque, for example during tip-in or other conditions. Further still, starter/generator and/or smart alternator systems may also be used. Moreover, additional torque may be provided via a suitable external energy source 8. The various components described above with reference to FIG. 1 may be controlled by a vehicle controller as will be described below with reference to FIG. 2.

From the above, it should be understood that the exemplary hybrid propulsion system is capable of various modes of operation. In a full hybrid implementation, for example, the propulsion system may operate using energy conversion device 6 (e.g., an electric motor) as the only torque source propelling the vehicle. This “electric only” mode of operation may be employed during braking, low speeds, while stopped at traffic lights, etc. In another mode, engine 2 is turned on, and acts as the only torque source powering drive wheel 4. In still another mode, which may be referred to as an “assist” mode, the alternate torque source 6 may supplement and act in cooperation with the torque provided by engine 2. As indicated above, energy conversion device 6 may also operate in a generator mode, in which torque is absorbed from engine 2 and/or transmission 3. Furthermore, energy conversion device 6 may act to augment or absorb torque during transitions of engine 2 between different combustion modes (e.g., during transitions between a spark ignition mode and a compression ignition mode).

Turning now to FIG. 2, a schematic diagram of an example engine 2 is shown, which may be included in a propulsion system of an automobile. The engine 2 is shown with four cylinders 30. However, other numbers of cylinders may be use in accordance with the current disclosure. Engine 2 may be controlled at least partially by a control system including controller 12, and by input from a vehicle operator 132 via an input device 130. In this example, input device 130 includes an accelerator pedal and a pedal position sensor 134 for generating a proportional pedal position signal PP. Each combustion chamber (e.g., cylinder) 30 of engine 2 may include combustion chamber walls with a piston (not shown) positioned therein. The pistons may be coupled to a crankshaft 40 so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft 40 may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system (not shown). Further, a starter motor may be coupled to crankshaft 40 via a flywheel to enable a starting operation of engine 2.

Combustion chambers 30 may receive intake air from intake manifold 44 via intake passage 42 and may exhaust combustion gases via exhaust passage 48. Intake manifold 44 and exhaust manifold 46 can selectively communicate with combustion chamber 30 via respective intake valves and exhaust valves (not shown). In some embodiments, combustion chamber 30 may include two or more intake valves and/or two or more exhaust valves.

Fuel injectors 50 are shown coupled directly to combustion chamber 30 for injecting fuel directly therein in proportion to the pulse width of signal FPW received from controller 12. In this manner, fuel injector 50 provides what is known as direct injection of fuel into combustion chamber 30. The fuel injector may be mounted in the side of the combustion chamber or in the top of the combustion chamber, for example. Fuel may be delivered to fuel injector 50 by a fuel system (not shown) including a fuel tank, a fuel pump, and a fuel rail. In some embodiments, combustion chambers 30 may alternatively, or additionally, include a fuel injector arranged in intake manifold 44 in a configuration that provides what is known as port injection of fuel into the intake port upstream from each combustion chamber 30.

Intake passage 42 may include throttles 21 and 23 having throttle plates 22 and 24, respectively. In this particular example, the position of throttle plates 22 and 24 may be varied by controller 12 via signals provided to an actuator included with throttles 21 and 23. In one example, the actuators may be electric actuators (e.g., electric motors), a configuration that is commonly referred to as electronic throttle control (ETC). In this manner, throttles 21 and 23 may be operated to vary the intake air provided to combustion chamber 30 among other engine cylinders. The position of throttle plates 22 and 24 may be provided to controller 12 by throttle position signal TP. Intake passage 42 may further include a mass air flow sensor 120 and a manifold air pressure sensor 122 for providing respective signals MAF (mass airflow) and MAP (manifold air pressure) to controller 12.

Exhaust passage 48 may receive exhaust gases from cylinders 30. Exhaust gas sensor 128 is shown coupled to exhaust passage 48 upstream of turbine 62 and an emission control device 78. Sensor 128 may be selected from among various suitable sensors for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO, a NOx, HC, or CO sensor, for example.

Emission control device 78 may be a selective catalytic reduction (SCR) system, a three way catalyst (TWC), NO_(x) trap, various other emission control devices, or combinations thereof. For example, device 78 may be an exhaust aftertreatment system which includes an SCR catalyst and a diesel particulate filter (DPF). In some embodiments, the DPF may be located downstream of the catalyst, while in other embodiments, the DPF may be positioned upstream of the catalyst. The DPF may be thermally regenerated periodically during engine operation. Further, in some embodiments, during operation of engine 10, device 78 may be periodically reset by operating at least one cylinder of the engine within a particular air/fuel ratio.

As shown, a urea injection system 82 is provided to inject liquid urea to SCR catalyst 71. The urea injection system 82 includes an injector 84, which is configured to inject a liquid reductant, such as a urea solution, into an exhaust gas flow path within exhaust passage 48. In the present embodiment, the injector 84 is angled relative to the exhaust passage 48. In alternate embodiments the injector may be either of parallel to or perpendicular to the exhaust passage. Further, the injector may include either air-assisted or hydraulic injection hardware. Urea injected into exhaust passage 48 may be converted to ammonia under certain conditions (e.g., in the presence of sufficient heat), which may then be used to reduce NOx in device 78 and/or may be stored in the device as described in further detail below.

Exhaust temperature may be measured by one or more temperature sensors located in exhaust passage 48, such as an exhaust gas temperature sensor 49. Alternatively, exhaust temperature may be inferred based on engine operating conditions such as speed, load, air-fuel ratio (AFR), spark retard, etc.

Controller 12 is shown in FIG. 2 as a microcomputer, including microprocessor unit 102, input/output ports 104, an electronic storage medium for executable programs and calibration values shown as read only memory chip 106 in this particular example, random access memory 108, keep alive memory 110, and a data bus. Controller 12 may receive various signals from sensors coupled to engine 2, in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor 120; engine coolant temperature (ECT) from temperature sensor 112, shown schematically in one location within the engine 2; a profile ignition pickup signal (PIP) from Hall effect sensor 118 (or other type) coupled to crankshaft 40; the throttle position (TP) from a throttle position sensor, as discussed; and absolute manifold pressure signal, MAP, from sensor 122, as discussed. Engine speed signal, RPM, may be generated by controller 12 from signal PIP. Manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum, or pressure, in the intake manifold 44. Note that various combinations of the above sensors may be used, such as a MAF sensor without a MAP sensor, or vice versa. During stoichiometric operation, the MAP sensor can give an indication of engine torque. Further, this sensor, along with the detected engine speed, can provide an estimate of charge (including air) inducted into the cylinder. In one example, sensor 118, which is also used as an engine speed sensor, may produce a predetermined number of equally spaced pulses every revolution of the crankshaft 40. In some examples, storage medium read-only memory 106 may be programmed with computer readable data representing instructions executable by processor 102 for performing the methods described below as well as other variants that are anticipated but not specifically listed.

Engine 2 may further include a compression device such as a turbocharger or supercharger including at least a compressor 60 arranged along intake manifold 44. For a turbocharger, compressor 60 may be at least partially driven by a turbine 62, via, for example a shaft, or other coupling arrangement. The turbine 62 may be arranged along exhaust passage 48. Various arrangements may be provided to drive the compressor. For a supercharger, compressor 60 may be at least partially driven by the engine and/or an electric machine, and may not include a turbine. Thus, the amount of compression provided to one or more cylinders of the engine via a turbocharger or supercharger may be varied by controller 12. In some cases, the turbine 62 may drive, for example, an electric generator 64, to provide power to a battery 66 via a turbo driver 68. Power from the battery 66 may then be used to drive the compressor 60 via a motor 70. Further, a sensor 123 may be disposed in intake manifold 44 for providing a BOOST signal to controller 12.

Further, exhaust passage 48 may include wastegate 26 for diverting exhaust gas away from turbine 62. In some embodiments, wastegate 26 may be a multi-staged wastegate, such as a two-staged wastegate with a first stage configured to control boost pressure and a second stage configured to increase heat flux to emission control device 78. Wastegate 26 may be operated with an actuator 150, which, for example, may be an electric actuator. In some embodiments, actuator 150 may be an electric motor. Intake passage 42 may include a compressor bypass valve 27 configured to divert intake air around compressor 60. Wastegate 26 and/or compressor bypass valve 27 may be controlled by controller 12 via actuators (e.g., actuator 150) to be opened when a lower boost pressure is desired, for example.

Intake passage 42 may further include charge air cooler (CAC) 80 (e.g., an intercooler) to decrease the temperature of the turbocharged or supercharged intake gases. In some embodiments, charge air cooler 80 may be an air to air heat exchanger. In other embodiments, charge air cooler 80 may be an air to liquid heat exchanger.

Further, in the disclosed embodiments, an exhaust gas recirculation (EGR) system may route a desired portion of exhaust gas from exhaust passage 48 to intake passage 42 via EGR passage 140. The amount of EGR provided to intake passage 42 may be varied by controller 12 via EGR valve 142. Further, an EGR sensor (not shown) may be arranged within the EGR passage and may provide an indication of one or more of pressure, temperature, and concentration of the exhaust gas. Alternatively, the EGR may be controlled through a calculated value based on signals from the MAF sensor (upstream), MAP (intake manifold), MAT (manifold gas temperature) and the crank speed sensor. Further, the EGR may be controlled based on an exhaust O₂ sensor and/or an intake oxygen sensor (intake manifold). Under some conditions, the EGR system may be used to regulate the temperature of the air and fuel mixture within the combustion chamber. FIG. 2 shows a high pressure EGR system where EGR is routed from upstream of a turbine of a turbocharger to downstream of a compressor of a turbocharger. In other embodiments, the engine may additionally or alternatively include a low pressure EGR system where EGR is routed from downstream of a turbine of a turbocharger to upstream of a compressor of the turbocharger.

As shown, an air pump 86 may be included and configured to inject air upstream emission control device 78, and may include an injector (not shown) operating via a mechanical coupling with a camshaft (not shown) of a cam actuation system (not shown). In one example, the mechanical coupling may include a rotating disc (not shown) configured to open a connection between the injector and exhaust passage 48 at certain times. In one embodiment, the air injector may be supplied with pressurized air via air pump 86, which may include on and off states regulated by controller 12. It is to be understood that air pump 86 and the air injector may be separate components or integrated into a single device.

FIG. 3 shows a flow chart illustrating a method 300 for controlling a level of ammonia stored in a catalyst. Method 300 may be stored on RAM 108 and executed via CPU 102 of controller 12 of FIG. 2, for example. The method is further described with reference to engine 10, urea injection system 82, and embodiments in which emission control device 78 is a catalyst (e.g., an SCR catalyst), though it will be appreciated that the method may be adapted for other suitable hardware.

At 302, it is determined whether the temperature of engine 10 is below a threshold (e.g., 100° C.) and further whether a level of ammonia currently stored in catalyst 78 is less than an ammonia capacity of the catalyst. The temperature of engine 10 may be determined as described above, for example based on the ECT signal provided via sensor 112. The ammonia capacity of catalyst 78 may be partially predetermined based on the physical characteristics of the catalyst and further determined during engine operation based on parameters including the temperature of the catalyst. The catalyst temperature may be determined via signals provided by a sensor including in catalyst 78 (not shown in FIG. 2) or alternatively determined based on exhaust gas temperatures sensed, for example, by sensor 49 of FIG. 2. The current level of ammonia stored in catalyst 78 may be determined based on the levels of NOx and urea flowing through the catalyst and their effective conversion rates, in part a function of temperature (e.g., exhaust gas temperature sensed via sensor 49). The levels of NOx and urea flowing through catalyst 78 may be detected, for example, via a NOx sensor sensitive to NOx and ammonia slip, positioned downstream the catalyst (not shown in FIG. 2).

If both the engine temperature is below the threshold and the ammonia stored in catalyst 78 is below its ammonia capacity (YES), method 300 proceeds to 304. If either or both of these conditions are not met (NO), the method ends.

At 304, urea is injected via urea injection system 82 upstream of catalyst 78. Here, catalyst 78 can be conditioned by introducing urea into the exhaust passage 48 in order to build up a store of ammonia in the catalyst. Urea injection may be performed during times at which engine 10 is substantially idle—e.g., when an associated vehicle is parked, since, when the engine is restarted or started when cold, the ammonia built up (e.g., accumulated) in the catalyst may be used for NOx reduction. Such an approach may be advantageous since the exhaust gas temperatures prevailing after a cold start are generally insufficient to evaporate an aqueous urea solution for the purpose of producing ammonia. To this extent, it is advantageous if sufficient ammonia is accumulated in the catalyst to be able to reduce NOx in a warm-up phase after restarting. It will be appreciated that the amount of urea injected at 304 may be based on both the current level of ammonia stored in catalyst 78 and the ammonia storage capacity of the catalyst, such that the level of ammonia stored in the catalyst following urea injection does not exceed the storage capacity. In this way, ammonia slip may be substantially prevented.

At 306, it is determined whether the temperature of catalyst 78 exceeds a first threshold. Ammonia stored in catalyst 78 may be released at temperatures above 400° C. without NOx reduction occurring. Accordingly, the first threshold may be a maximum permissible temperature which the catalyst 78 is not allowed to exceed, and may be, in some embodiments, between 350° C. and 450° C. In other embodiments, the maximum permissible temperature may be between 370° C. and 430° C., and in yet other embodiments, between 380° C. and 420° C. If it is determined that the temperature of catalyst 78 does not exceed the first threshold (NO), method 300 proceeds to 310. If the catalyst temperature does exceed the first threshold (YES), method 300 proceeds to 308.

At 308, the load of engine 10 is reduced. Load reduction may include one or more actions, including enleaning the air/fuel mixture supplied to cylinders 30 of engine 10 and deactivating the engine 10. Engine 10 may be deactivated, for example, by blocking the supply of FPW signals described above, deactivating spark plugs associated with cylinders 30 in embodiments in which engine 10 is spark-ignited, etc. In some embodiments, upon deactivating engine 10, energy conversion device 6 of FIG. 1 is operated to drive engine 10 in order to pump fresh air through catalyst 78. Here, engine 10 acts as a piston-type machine—e.g., as a pump which, driven by device 6, pumps fresh air through the catalyst. In this way, heat is withdrawn from the catalyst by convection, thereby cooling the catalyst.

Energy conversion device 6 may be further configured to drive a secondary air pump such as air pump 86 of FIG. 2 in order to pump fresh air through catalyst 78. As similarly described above, such action may lower the temperature of catalyst 78 via convection. The secondary air pump may be operated to lower the catalyst temperature regardless of whether engine 10 is deactivated or not.

Actions taken at 308 to reduce the load of engine 10 may further include increasing the rate of exhaust gas recirculation supplied to cylinders 30 via EGR passage 140. As the engine load falls, it is not only the exhaust gas temperatures and the exhaust gas volume flows which decrease. The absolute exhaust gas mass passed through catalyst 78 often decreases as well in practice since, as the load decreases, an increasing exhaust gas quantity is generally recirculated to reduce the untreated NOx emissions, and this then no longer flows through the catalyst. Exhaust gas recirculation is a concept for lowering NOx emissions, in which the NOx emissions can be significantly lowered as the exhaust gas recirculation rate increases. The exhaust gas recirculation rate x_(AGR) may be given by X_(AGR)=m_(AGR)/(m_(AGR)+m_(FA)), where m_(AGR) denotes the mass of recirculated exhaust gas and m_(FA) denotes the fresh air supplied. In order to achieve a significant reduction in NOx emissions, high exhaust gas recirculation rates may be applied, for example on the order of X_(AGR)≈60% to 70%.

If exhaust gas is recirculated in order to reduce untreated NOx emissions, the absolute exhaust gas mass passed through catalyst 78 decreases, with the result that the catalyst dwell time relevant for conversion increases. Further, the exhaust gas may have a lower NOx concentration due to exhaust gas recirculation. Both of these factors lead to a reduction in the quantity of pollutants contained in the treated exhaust gas and introduced into the environment.

At 310, it is determined whether the temperature of catalyst 78 falls below a second threshold. Evaporation of aqueous urea and conversion to ammonia may occur at exhaust gas temperatures approximately between 150° C. and 170° C. Accordingly, the second threshold may be a minimum permissible temperature which catalyst 78 is prevented from falling below, and may be in some embodiments between 120° C. and 200° C. In other embodiments, the minimum permissible temperature may be between 140° C. and 180° C., and in yet other embodiments, between 150° C. and 170° C. If it determined that the temperature of catalyst 78 does not fall below the second threshold (NO), method 300 proceeds to 314. If it determined that the temperature of catalyst 78 does fall below the second threshold (YES), method 300 proceeds to 312.

At 312, the load of engine 10 is increased, which may include various actions. For example, the air/fuel mixture supplied to cylinders 30 may be enriched. Further, the exhaust gas recirculation rate supplied to the cylinders may be reduced.

In this way, exhaust gas temperatures and catalyst temperatures may be maintained within acceptable limits, ammonia slip may be substantially prevented, and in some scenarios, engine 10 may be maintained in a medium load range. The medium load range may be, for example, between 30% and 70% of the maximum load of the engine. In some embodiments, high and low load regions may be substantially and respectively centered about these values.

At 312, it is determined whether the torque demanded by a vehicle operator (e.g., driver-demanded torque) has been overshot or undershot. A tolerance may be provided, however, such that small deviations between supplied torque and driver-demanded torque may not prompt compensating action. The tolerance may be, for example, 2%. If the driver-demanded torque has not been overshot or undershot (NEITHER), method 300 ends. If the driver-demanded torque has been undershot (UNDERSHOT), the method proceeds to 316 where the torque output of a motor (e.g., energy conversion device 6 of FIG. 1) operatively coupled to engine 10 is adjusted to increase the overall supplied torque and meet the driver-demanded torque. If instead the driver-demanded torque has been overshot (OVERSHOT), method 300 proceeds to 318 where the torque output of the motor is adjusted to reduce the overall supplied torque, operating for example in a generator mode by absorbing excess torque as described above. Torque reduction of engine 10 due to load adjustment is thus compensated for at 316 and 318, and sufficient ammonia supply and storage is ensured without disrupting the driving experience. Following 316 and 318, method 300 ends.

It will be appreciated that in some embodiments, load reduction and increase of engine 10 at 308 and 312, and consequently the actions taken to perform such load adjustments (e.g., enleaning/enriching the air/fuel ratio, EGR rate adjustment, etc.) may be performed in proportion to the stored ammonia level in catalyst 78 such that the stored ammonia level remains within the ammonia capacity of the catalyst. Load adjustment of engine 10 may be carried out to the extent that the level of ammonia stored in catalyst 78 remains within its ammonia capacity, saving power and energy. FIG. 4 illustrates such an approach.

FIG. 4 in particular shows an exemplary graph 400 of the ammonia storage capacity 402 of catalyst 78 as function of temperature (e.g., catalyst temperature). For a particular operating point at a temperature T₀, the stored ammonia level 404 in catalyst 78 exceeds the ammonia storage capacity 402 of the catalyst. Accordingly, the load of engine 10 is reduced in the manner described above and in proportion to the stored ammonia level, in turn reducing temperatures to a temperature T₁ to the extent required to bring the stored ammonia level 406 at a subsequent time within the ammonia storage capacity 402 of catalyst 78. The load of engine 10 may be increased in proportion to the stored ammonia level in a similar manner.

At another operating point at a relatively lower temperature T₂, the stored ammonia level 408 in catalyst 78 exceeds the ammonia storage capacity 402 of the catalyst. The load of engine 10 is reduced in proportion to stored ammonia level 408, reducing temperatures to bring the stored ammonia level 410 within ammonia storage capacity 402 of catalyst 78 at a relatively lower temperature T₃. The temperature reduction performed for the pair of stored ammonia levels 408 and 410 is relatively higher compared to that performed for the pair of stored ammonia levels 404 and 406.

In contrast, the stored ammonia level 412 at a relatively lowest temperature T₄ does not prompt temperature or load adjustment as this level is within the ammonia storage capacity 402 of catalyst 78. As such, engine 10 is operated normally, and in response to, for example, driver-demanded torque.

FIG. 4 also illustrates how a stored ammonia level 414 at temperature T₀ prompts a greater amount of load reduction compared to stored ammonia level 404 at the same temperature. For the relatively greater stored ammonia level 414, the load of engine 10 is reduced to a relatively greater extent, yielding stored ammonia level 416 at a comparatively lower temperature T_(0,f). As described above, depending on the driver-demanded torque requested during this load reduction, a greater amount of supplemental motor torque may be supplied by energy conversion device 6 of FIG. 1 relative to the supplemental motor torque supplied when reducing the engine load to lower stored ammonia level 404 to stored ammonia level 406. FIG. 4 further illustrates how a stored ammonia level 420 at temperature T₀ does not prompt any load reduction at all, as it is below the storage capacity curve. Thus, for the same temperature T₀, depending on the relative values of the actual stored ammonia level and the storage capacity, different degrees of load reduction, or no load reduction at all, may be used.

It will be appreciated that graph 400 is provided as an illustrative example and is not intended to be limiting in any way. Actual graphs of stored ammonia as a function of temperature will vary depending on operating conditions and characteristics of an engine and an associated catalyst.

Application of method 300 of FIG. 3 may present several advantages. As exhaust gas temperatures may be maintained below an upper limit, with energy conversion device 6 of FIG. 1 compensating for reduced engine loads and driver-demanded torque, lower exhaust gas temperatures may cause reduced exhaust gas volume flows. The lower exhaust gas volume flows permit a catalyst of smaller dimensions (e.g., reduced volume) without a reduction in the space velocity relevant to conversion, reducing cost. The smaller volume also enables the catalyst to be arranged closer to the engine without exhaust gas backpressure assuming or exceeding impermissible values. This latter point has advantages, in particular, regarding heating of the catalyst after a cold start.

Method 300 may also be adapted for alternative or additional entry conditions. For example, the NOx concentration or ammonia concentration downstream of the catalyst 78 may be used to initiate the method. An impermissibly high NOx concentration or ammonia concentration may indicate an excessive catalyst temperature or a degraded condition of the catalyst.

Note that the example control and estimation methods included herein can be used with various engine and/or vehicle system configurations. The specific methods described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts may graphically represent code to be programmed into the computer readable storage medium in the engine control system.

It will be appreciated that the configurations and methods disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure. 

1. A method for operating an internal combustion engine, comprising: controlling a load of the internal combustion engine based on an exhaust gas temperature of the engine; wherein an energy conversion device drivingly coupled to the engine is configured to absorb power from the engine and output additional power to the engine, wherein the load of the engine is reduced if a temperature of at least one catalyst of the engine exceeds a maximum permissible temperature, and wherein the energy conversion device is operated as a selectable auxiliary drive to satisfy a requested additional power demand.
 2. The method of claim 1, wherein the internal combustion engine is deactivated if the temperature of the at least one catalyst exceeds the maximum permissible temperature, and wherein the energy conversion device is operated to satisfy the requested additional power demand.
 3. The method of claim 2, wherein the energy conversion device is operated to drive the internal combustion engine in order to pump fresh air through the at least one catalyst.
 4. The method of claim 1, wherein the internal combustion engine is deactivated if the temperature of the at least one catalyst exceeds the maximum permissible temperature, and wherein the energy conversion device is operated to drive a secondary air pump in order to pump fresh air through the at least one catalyst.
 5. The method of claim 1, wherein the maximum permissible temperature is between 350° C. and 450° C.
 6. The method of claim 1, wherein the load of the internal combustion engine is increased in order to raise the exhaust gas temperature and hence the temperature of the at least one catalyst; and wherein the energy conversion device is operated as a selectable generator in order to absorb excess power provided by the engine.
 7. The method of claim 1, wherein the load of the internal combustion engine is increased if the temperature of the at least one catalyst falls below a minimum permissible temperature; and wherein the minimum permissible temperature is between 120° C. and 200° C.
 8. The method of claim 1, wherein the internal combustion engine is operated in a medium load range, the energy conversion device operated as a selectable auxiliary drive to satisfy the requested additional power demand if a high power is demanded; and wherein the energy conversion device operated as a selectable generator to absorb excess power provided by the engine if a low power is demanded.
 9. The method of claim 8, wherein the medium load range includes loads between 30% and 70% of a maximum load of the internal combustion engine at a given engine speed.
 10. The method of claim 1, wherein ammonia is used as a reductant; and wherein the internal combustion engine is operated such that a sufficiently large store of ammonia is built up in the at least one catalyst for restarting to reduce nitrogen oxides in a warm-up phase.
 11. A method, comprising: adjusting an amount of supplemental motor torque in response to an amount of ammonia stored relative to a storage capacity of an SCR catalyst coupled to an internal combustion engine.
 12. The method of claim 11, wherein the amount of supplemental motor torque is adjusted if a temperature of the SCR catalyst exceeds a first threshold at which the storage capacity of the catalyst for the temperature is below the amount of ammonia stored, while reducing a load of the engine.
 13. The method of claim 12, wherein the amount of supplemental motor torque is adjusted to supply additional torque if a driver-demanded torque is undershot after reducing the load of the engine; and wherein the amount of supplemental motor torque is adjusted to reduce torque if the driver-demanded torque is overshot after reducing the load of the engine.
 14. The method of claim 12, wherein reducing the load of the engine includes deactivating the engine.
 15. The method of claim 12, wherein reducing the load of the engine includes enleaning an air/fuel mixture.
 16. The method of claim 11, wherein the amount of supplemental motor torque is adjusted if a temperature of the SCR catalyst falls below a second threshold at which urea substantially evaporates, thereby increasing a load of the engine.
 17. The method of claim 16, wherein the amount of supplemental motor torque is adjusted to supply additional torque if a driver-demanded torque is undershot after increasing the load of the engine; and wherein the amount of supplemental motor torque is adjusted to reduce torque if a driver-demanded torque is overshot after increasing the load of the engine.
 18. The method of claim 16, further comprising, if the load of the engine is in a low region, and the amount of ammonia stored is below the storage capacity of the catalyst, injecting urea upstream of the catalyst.
 19. A method, comprising: reducing a load of an internal combustion engine in proportion to an amount of ammonia stored relative to a storage capacity of an SCR catalyst; while adjusting an amount of supplemental motor torque, based on exhaust temperature.
 20. The method of claim 19, wherein the load of the engine is reduced in proportion to the stored ammonia level in the catalyst such that stored ammonia level is maintained below the ammonia capacity. 